Multiple foundation species shape benthic habitat islands
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Pattern generation by foundation species (FS) is a primary structuring agent in marine and terrestrial communities. Prior research, focused on single-species or guild-dominated habitats, stressed the role of facilitation in maintaining community structure. However, many habitats are developed by multiple FS from different guilds. Competition between these FS may provide an additional agent potentially responsible for spatial and temporal patterns. In the White Sea, epibenthic patches formed by barnacles (Balanus crenatus) and solitary ascidians (mainly Styela spp. and Molgula spp.) on small stones and empty bivalve shells (mainly Serripes groenlandicus) produce microhabitats for different sessile taxa. We hypothesized that: (1) several FS would provide habitats for most of other species in the community; (2) different FS promote different assemblages of sessile organisms; (3) the interplay of facilitation and competition best explains observed patterns of abundance and demography in FS; and (4) these interactions shape the whole community, increasing the diversity compared to less heterogeneous patches constituted by single FS. We examined 459 patches and the results generally supported this hypothesis. The number of FS in a patch positively affected species diversity. Most sessile species (72% of individuals) resided on barnacles, ascidians and red algae, except barnacles that dominated the primary substrate. The size structure of barnacles (live individuals and empty shells) and ascidians were interrelated, suggesting long-term patch dynamics whereby ascidians regularly replace barnacles. Following this replacement, we expect consequent changes to the entire dependent assemblage. Evidence for these changes exists in the spatial pattern: most sessile and motile taxa demonstrated significant associations with either FS. Our results indicate that the small-scale patterns observed in patches formed by multiple FS are primarily generated by facilitation of dependent taxa by FS, and facilitation and competition between different FS.
KeywordsFacilitation Competition Habitat Barnacles Ascidians
We thank our colleagues Alexey Grishankov [St Petersburg State University (Spb)] and Michael Fokin (Zoological Institute RAN, SPb) for their collaboration and fruitful discussions. Some earlier ideas by Daniel Alexandrov (European University in SPb) contributed to our inspiration. We acknowledge numerous volunteers that shared the ups and downs of our field life. Technical support and accommodation was provided by the Biological Station of Moscow State University. Our special heartfelt thanks to Alexander and Nadezhda Cherenkovy. The success in our diving could only be achieved with the help of Dmitry Tomanovskiy and the Polar Institute for Fisheries and Oceanography. Thanks are due to Sergey Dobretsov (University of Kiel) for his assistance in data processing, to Judi Hewitt (NIWA, Hamilton, New Zealand), Alexey Koupriyanov (European University in SPb), Natalia Lenstman (SPb State University) and Mark C. Urban (Yale University) for discussion and linguistic corrections. We acknowledge Statsoft for a copy of STATISTICA software package granted to SPb State University. Tony Underwood (University of Sydney) and four anonymous reviewers made valuable comments on the earlier versions of the manuscript. Financial support was provided by RFBR (grant nos. 02-04-50020A, 05-04-48927A, 05-04-63041K, 06-04-63077K, 07-04-10075K), Universities of Russia program (grant no. UR-0701013), ISSEP (grant nos. s96-837 and s97-1711).
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